A wavelength division multiplexer/demultiplexer can be fabricated from a fiber optic Mach-Zehnder interferometer (MZI). The MZI is fabricated from a pair of symmetric couplers. Identical fiber Bragg gratings are written in the interfering arms between the couplers. A fiber Bragg grating (FBG) is a change in the refractive index in the fiber core that reflects a selective wavelength on the fiber.
In operation of the MZI to drop or extract a wavelength, for example, as a demultiplexer, a signal carrying several channels or wavelengths .lambda.1, .lambda.2, .lambda.3, .lambda.4, and .lambda.5 is input into the first fiber of the first coupler. Other numbers of channels, for example, 4 or 8 channels, may be input into the coupler. The signal is split at the coupler to pass along both arms. The FBGs, which are identical, are resonant at a selected frequency, for example, .lambda.4. Thus, at the FBGs, .lambda.4 is reflected, passes back through the first coupler, and is extracted on the second fiber of the first coupler. The remaining wavelengths, .lambda.1, .lambda.2, .lambda.3, and .lambda.5, pass through the second coupler and are output on the second fiber of the second coupler.
In operation to add or insert a wavelength, for example, as a multiplexer, a signal having the wavelength .lambda.4 is inserted on the first fiber at the second coupler. A signal of various wavelengths .lambda.1, .lambda.2, .lambda.3, and .lambda.5, is input into the first fiber of the first coupler. As described above with respect to the demultiplexer, .lambda.4 is reflected at the FBGs. .lambda.4 is then output on the second fiber of the second coupler. Thus, the output of the second coupler includes all the wavelengths, .lambda.1 , .lambda.2, .lambda.3, .lambda.4, and .lambda.5.
In typical Mach-Zehnder interferometers fabricated from symmetric couplers, 30 dB isolation is limited to spans of .+-.20 nm about the desired wavelength.